The present invention relates to steam turbines for electric power plants and control systems for such turbines, and more particularly to digital turbine control systems having a novel speed loop selection arrangement.
In the operation of the steam turbines, the single most important variable which must be controlled is shaft speed. During load operation, speed regulation is necessary to help the powder system maintain the line frequency. During wide range speed control, precise speed control is doubly important to bring the unit to synchronous speed, and to overcome critical speed points at which excessive vibrations may cause a turbine trip. To accomplish such demanding control objectives, it is necessary to provide high-accuracy speed input signals to the control system so that exact valve position outputs may be computed by the speed controllers.
In an installation where the control system, at least, in the automatic mode is performed by a digital computer, it is desirable to have a plurality of independent speed, signals both analog and digital form for determining the speed of the turbine. In utilizing such signals, it is also desirable to provide a means for determining the validity of such signals, with the system responding only to the valid signal.